Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1H, there are cross sectional views illustrating a method for manufacturing an array 100 of thin film actuated mirrors 101 previous disclosed for use in an optical projection system.
The process for the manufacture of the array 100 begins with the preparation of an active matrix 110 including a substrate 112 and an array of connecting terminals 114. The substrate 112 is made of an insulating material, e.g., Si-wafer, and the connecting terminal 114 is made of a conducting material, e.g., tungsten (W), as shown in FIG. 1A.
In a subsequent step, there is formed a passivation layer 120, made of, e.g., PSG or silicon nitride, and having a thickness of 0.1-2 .mu.m, on top of the active matrix 110 by using, e.g., a CVD or a spin coating method.
Thereafter, an etchant stopping layer 130, made of silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the passivation layer 120 by using, e.g., a sputtering or a CVD method, as shown in FIG. 1B.
Then, a sacrificial layer 140, made of a PSG and having a flat top surface, is formed on top of the etchant stopping layer 130 by using a CVD or spin coating method, followed by a chemical mechanical polishing (CMP) method.
Subsequently, an array of empty cavities 145 is created in the sacrificial layer 140 in such a way that each of the empty cavities 145 encompasses one of the connecting terminals 114 by using a dry or an wet etching method, as shown in FIG. 1C.
In a next step, an elastic layer 150, made of a nitride, e.g., silicon nitride, and having a thickness of 0.1-1 .mu.m, is deposited on top of the sacrificial layer 140 including the empty cavities 145 by using a CVD method.
Thereafter, a lower electrode layer 160, made of an electrically conducting material, e.g., Pt or Ta, and having a thickness of 0.1-1 .mu.m, is formed on top of the elastic layer 150 by using a sputtering or a vacuum evaporation method, as shown in FIG. 1D.
Then, an electrodisplacive layer 170, made of a piezoelectric material, e.g., PZT, and having a thickness of 0.1-1 .mu.m, is formed on top of the lower electrode layer 160 by using a sol-gel method.
Subsequently, an upper electrode layer 180, made of an electrically conducting and light reflecting material, e.g., aluminum (Al) or silver (Ag), and having a thickness of 0.1-1 .mu.m, is formed on top of the electrodisplacive layer 170 by using a sputtering or a vacuum evaporation method, thereby forming a multiple layered structure 200, as shown in FIG. 1E.
In an ensuing step, as shown in FIG. 1F, the multiple layered structure 200 is patterned by using a photolithography or a laser trimming method, until the sacrificial layer 140 is exposed.
In a subsequent step, an array of conduits 190, made of a metal, e.g., tungsten(W), is formed by using a lift-off method thereby forming an array of actuating structures 210, wherein each of actuating structures 210 includes an upper electrode 185, an electrodisplacive member 175, a lower electrode 165, an elastic member 155 and a conduit 190, the conduits 190 extending from the lower electrode 165 to a corresponding connecting terminal 114, as shown in FIG. 1G.
Finally, the sacrificial layer 140 is removed by using an wet etching method using an etchant or a chemical, e.g., hydrogen fluoride (HF) vapor, to thereby form an array 100 of thin film actuated mirrors 101, as shown in FIG. 1H.
There are certain deficiencies associated with the above described method for the manufacture of the array 100 of thin film actuated mirrors 101. For example, in order to obtain optimum piezoelectric properties in the piezoelectric material, e.g., PZT, constituting the electrodisplacive layer 170, grains must grow in certain direction, e.g., &lt;111&gt;. However, when the electrodisplacive layer 170 is formed using the sol-gel method, there is a likelihood of the grains not growing the required direction, resulting in the piezoelectric material not having the optimum piezoelectric properties.
Further, when the electrodisplacive layer 170 is thus formed, grains therein may also become large, causing the top surface of the electrodisplacive layer 170 to become rough. When the upper electrode layer 180 functioning as a mirror is consequently formed on top thereof, the top surface of the upper electrode layer 180 may also become rough, which will, in turn, detrimentally affect the optical efficiency of the array.